mathjax

2015-10-08

So one day I decided that I wanted to make a cutesy little bench top supply because bench top supplies are balls expensive. I'm going to talk about the rest of this venture at a later date. But today I will talk for too long about the gate drive and how it is for the most part an exercise in curiosity.

Imagine for a moment that you wanted to switch a supply at 500kHz on a full bridge in an odd synchronous fashion. Your are strapped by cash and need synchronous rectification on the output because you're going to be dealing with high current and low voltages. You also want the output floating because if the output floats that's handy it means you can have +V or -V or stacked or paralleled if the outputs can share current.

But doing synchronous rectification requires gate drives, some weird boot strapping some form of isolation. If gate drives go in that means an auxiliary floating set of voltage rails for the drivers and some sort of sensing the voltage state which means chips and rails and money and stuff.

It would be nice if all the switches were driven by floating things, you know kinda like a transformer.

But transformers cost money they are big and slow and inductive... but that is a fallacy. Lots of fast things use transformers for signals, pulse transformers are things, USB uses transformers, ETHERNET uses transformers. Ain't nobody calling my gigabits slow. There are some highly legitimate reasons why signals like usb and ethernet utilize magnetic coupling for signals:

It can be isolated electrically. My laptop could sit at a completely different potential than your laptop, powered by batteries or off the line and they could still yarble at one another over an CAT5e cable thanks tot he awesomeness that is transformers.

A local return path/ reference. Any signal sent out needs to be with reference to something and have a return path and to be a fast signal inductance needs to be minimized. This is why PCIe, usb, ethernet, SATA and ect. are all differential pairs for high speed signalling. Transformers by nature create a differential voltage source. As a statement of Gauss's law and conservation of charge they must create a + and relative - to maintain the E field flux but I'm going off on a limb. The important bit is that by creation of a differential signal a return path exists that does not necessarily need to be referenced to ground or anything in particular besides the other end of the wire in the transformer.

You can have common modes impedance without differential mode impedance. if your output if flapping around a some voltage this can help keep noise from getting into the primary side of the circuit. because transformers aren't perfect and they have some amount of capacitance going from the secondary to primary side.

Transformers are pretty nifty for a variety of reasons, but they have their limitations. That being said in the system of this gate drive I am worried about the volt-seconds that the cores will be able to withstand before saturating.

Once the cores become saturated the primary becomes decoupled from the secondary in at least one direction and the ability to keep the switches in a specific state compromised (to some extent). The ethernet transformers I have seen largely don't come with a primary volt-second spec. However the ethernet transformers I have seen seem to posses a magnetizing inductance of ~300uH which I thought was surprisingly high, and I think puts the idea of the gate driver on the edge of potentially acceptable/ practical.

As far as practical implementations go all of the ethernet transformers are center tapped on the primary and secondary side. By running a push-pull converter using the primary centertap the transformer is a 2:1 step up. This allows you to effectively drive a fet using logic level voltages, once you get above 10V on the gate of a FET the reduction in Rds on is generally marginal. I think the advantage this system would get from running at higher voltages is the faster turn on time of the switch but once the switches are on it won't matter much.

This brings into discussion a few things which I have marginally modeled in spice. What limits the turn on speed of the device? Like I got a voltage source with some impedance. The switches require some base line amount of voltage to turn on. The FET gates are a derpy capacitor, in order to turn on the switch requires some amount of charge slapped on the gate which roughly translates into a given amount of energy (roughly). The goal of a fast gate driver must be to deposit that energy onto the gate as a fast as possible. Any impedance between your roughly ideal voltage source of a decoupling cap which is hopefully sized significantly larger than your gate capacitance otherwise what the fuck are you doing, will restrict the flow of power. There were many many assumptions made in that last sentence.

But what it boils down to is what is throttling the current? resistance or inductance? with a transformer inductance can be a severe issue at high frequency. Even if the fundamental frequency of the switching isn't that fast what is important its about the rise time of the gates voltage waveform. The long the turn on the longer the losses and that what I care about reducing.

But holy fuck the post is longer than I expected and I'm just glossing over things. may I'll post the spice sim next post.